Radiant exitance (or emittance) is a term of radiometry and is defined as the radiant flux (optical power = energy per unit time) which is emitted by some surface (e.g. of a light source) per unit area.
In the SI system, the units of the radiant exitance are W/m2 (watts per square meter).
The related term irradiance has the same units, but applies to received radiation.
A related quantity is the spectralexitance, which is the exitance per unit frequency or wavelength interval.
It has units of W / (m2 Hz) or W / (m2 nm), for example.
The corresponding photometric quantity is the luminous exitance.